1. Field of the invention
The present invention relates to a curtain coater for coating a layer of liquid coating composition on a continuous web in the manufacture of a photographic element.
2. Description of the prior art
In the manufacture of a coated photographic element, the coating compositions typically consist of aqueous solutions or dispersions containing hydrophilic colloids with or without other materials dissolved or dispersed therein. They are liquid compositions of relatively low viscosity, for example, of less than about 150 cP (centipoise), most in the range from about 5 to about 100 cP. After having been coated onto the surface of a support they are subjected to controlled temperatures to effect setting and drying. In the photographic art coating compositions are used which differ very much in chemical composition and, to a more limited extent, in physical characteristics. Various materials are used as the support. Thus, for example, the support is made of paper, film base, glass, cloth and the like, and it may be coated in the form of discrete sheets or, as is more usual, in the form of a continuous web.
The manufacture of photographic elements is a tremendously difficult art requiring extremely accurate control. Unlike coating operations in other arts, where complete coverage of the article being coated and attractive appearance are usually the only essentials for any particular coating method, in the photographic art the coating method must provide for precise control. A photographic element requires coating of individual layers which are extremely thin, i.e. a maximum wet thickness of about 150 micrometers, and generally far below this value, e.g. as low as about 10 micrometer. After coating the layers have to be set and/or dried before the product can be handled and their surfaces generally cannot be subjected to any physical treatment to increase their smoothness and/or their thickness uniformity. For this reason, the coating composition must be applied to the support in such a precise manner that after the layer is set and/or dried it will be within permissible tolerances with respect to both thickness and uniformity. Since an individual layer must be extremely thin, as is indicated above, and since the maximum variation in thickness uniformity is mostly plus or minus a few percent, it is obvious that the coating operation in the manufacture of photographic elements is an unusually complex and demanding procedure. Moreover, the difficulties involved in meeting the requirements of utmost thinness and extreme uniformity are magnified by the fact that in order to be commercially practical, the coating operation must be capable of handling continuous webs with a width up to one meter or more and must enable the web to be coated at high speeds, for example, speeds as high as several meters per second.
A particularly useful coater for realizing the objects set forth hereinbefore is a curtain coater. If such apparatus is arranged to provide exact control of the means by which the free-falling curtain is generated, and of certain critical relationships between the operating variables, high quality photographic elements may be produced with this type of coater. Basic patents on the use of a certain coater for the production of photographic elements are U.S. Pat. No. 3,632,374 and U.S. Pat. No. 3,508,947 relating to a single-layer and to a multiple-layer curtain coater respectively.
A phenomenon observed at coating speeds higher than approximately 150 m.min.sup.-1 is the displacement of the curtain in the direction of the web movement by the air entrained by the web. A small layer of air that sticks to the moving web hits the contact line between curtain and web. Moreover the displacement of this contact line is not uniform since the curtain assumes a wavelike or undulating deformation, when viewed laterally of the curtain. As a consequence of the curtain deformation, the coated layer exhibits longitudinal bandlike thickness deviations. These bandlike deformations are of the order of magnitude of a few percent only, and are mostly not disturbing in the case of opaque photographic materials that are viewed or used in reflection. In the case, however, of photographic materials that are viewed in transmission or projection, the density variations caused by bandlike thickness variations in one or more of its light-absorbing layers, whether these layers are light-sensitive or not, are unacceptable.
In order to avoid this problem, one has to remove the boundary layer of air from the surface of the web. It has been proposed to obviate the problem is question by means of different techniques.
First, it is known to provide the coater with shield means that extend parallel with the curtain and terminate in close proximity of the web surface, with an end portion deflected in countercurrent direction relative to web movement. The shield means may occasionally be provided with a vacuum manifold operatively connected thereto for evacuating air from the surface of the web. The described improvement is disclosed in U.S. Pat. No. 3,867,901. We have noticed that at speeds over 150 to 200 m.min.sup.-1, depending on the thickness of the applied layer, this kind of shield means does not prevent the formation of bands in the coated layer.
Another arrangement for removing the boundary layer of air from a web in a curtain coater is disclosed in FR-A 1,463,674. In this patent specification, which does not explicitly relate to the manufacture of photographical elements, a coater is described in which a web (such as cardboard or cellulose derivatives) is transported through a coating curtain by means of a conveyor roller before and after the curtain, and in which the web is deflected slightly downwards by contact with a knife edge that forms an air-tight joint between the knife and the web at a certain distance upstream of the curtain. According to an alternative embodiment of the arrangement, the knife is hollow and has an open edge at its underside, whereby the entrained air may be sucked off. Although the knife effectively removes the boundary layer of air from the web and also stabilizes the curtain as well as the web, its use is excluded in the manufacture of photographic elements since the frictional contact with the support inevitably damages the surface of the support. Damaging of a delicate web also occurs by particles of dust and the like that become collected at the front side of the knife and that cause scratching of the web surface.
Positioning this knife above the web as in FIG. 9 of U.S. Pat. No. 3,632,374 to avoid any contact, requires the use of large flowrates of sucked-off air in order to remove the boundary layer sufficiently. However, it is practically impossible to get uniform evacuation, across the width of the web of the boundary layer when flowrates of this order of magnitude are used. Any non-uniformity causes bandlike disturbances in the coated layer.
A still further arrangement for removing a boundary layer of air from a web comprises a concave shielding plate that is curved concentrically about the axis of a web-supporting roller and spaced from the roller periphery a distance not larger than about 1 mm. The narrow gap that is formed between the air shield and the web on the roller forms an important resistance to air entrained with the web, and thereby allows the use of higher coating speeds. The mentioned arrangement is disclosed in Research Disclosure No. 18916 of January 1980, but also with this arrangement a practical upper limit of the coating speed is approximately 200 m.min.sup.-1 for a shield spacing of 1 mm. Due to constructive limitations smaller shield spacings can be used only for smaller curtain widths, such as curtain widths smaller than about 40 cm.
Finally, in DE-B-1,269,546 is disclosed a curtain coater in which individual objects to be coated are transported through a coating zone by means of two endless belts. Disturbing influences of air displacements in the coating room and of air entrained by the objects are reduced by brushes that bear on the end of a straight advancing stretch of the first belt. The effect of the described measure is also limited and in fact is advantageous only for the types of coating that are disclosed in the cited document, namely paints and adhesives.
It is clear that brushes with bristles or hairs that are stiff and/or sharp-ended are not suited for use in manufacturing of photographic material. It is even possible that the brush catches and gathers dust particles, and finally that large agglomerates of such particles drop free of the bristles and slip under the brush. Such agglomerates then become wound between successive convolutions of the roll of web and cause a permanent defect in the web surface. Positioning the brush above the web without making contact is an embodiment which avoids the problem of gathering the dust particles but at the price of unsatisfactory removal of the boundary layer.